1. Field of the Invention
The present invention relates to a method of treating clay to brighten it by the removal of discoloring impurities, in particular titanium mineral impurities; e.g. TiO.sub.2 impurities, and more particularly relates to a highly efficient method involving the use of froth flotation procedures and oxidizing and reducing bleaches as well as a polyacrylate salt deflocculant employed at the most effective point in the method.
2. Prior Art
Kaolin clay as mined is impure, being mechanically associated with a variety of other minerals. Pure kaolinite is a hydroxyl-containing aluminosilicate mineral and is comprised of the elements hydrogen, oxygen, aluminum, and silicon. In kaolin clay, in addition to these elements, there are other elements of which carbon, magnesium, calcium, sodium, potassium, iron, and titanium are but a few of the two dozen or so elements that can be detected by chemical analysis. If pure kaolinite is colorless, the observed discolorations in kaolin must be attributed to the impurity elements that are found either in an inorganic or organic form with the clay. These very finely divided discoloring impurities greatly detract from the value and usefulness of the clay for many applications such as in the making of high quality paper, where a pigment that has a high brightness and whiteness is required. Impure titanium dioxide (TiO.sub.2) is recognized to be an extremely important discoloring impurity in kaolin; presumably iron becomes incorporated into the crystalline lattice of the anatase and rutile forms of titanium dioxide to give a highly colored pigment. As little as 1% iron or less in anatase or rutile will render it highly colored. As used herein, the term "titanium dioxide" refers to the impure form of anatase and rutile.
Processes for separating and concentrating ores and minerals into components have included two general approaches; direct chemical attack on one or more of the components of the minerals and physical processes that achieve selective separation of one or more of the components from the minerals and ores. With kaolin, direct chemical attack has not proven to be completely effective and economically feasible. Conventional bleaching processes employing weakly acidic, strong reducing conditions limit the brightness gain for kaolin clay, since the TiO.sub.2 content remains essentially unaffected.
Various methods of physically separating and concentrating minerals and ores are known in the art, including froth flotation systems. A prerequisite of all such methods is that the feed material be crushed or ground to a degree of fineness such that mechanical interlocking between the various mineral components present has been eliminated. Component separation of the mineral and ores can then be performed by preparing a slurry or suspension of the clay in water with the aid of dispersing agents. The prior art on froth flotation processes calls for adjusting the pH of the slurry to either an acidic or alkaline pH value, adding a collector, conditioning the slurry for a time to dissipate energy into the clay, adding frothing agents to the conditioned slurry, diluting the slurry to low solids, increasing the pH above 8, and passing air through the slurry. A modification of the conventional practice of froth flotation is commercially called "Ultraflotation" and comprises adding a finely divided carrier mineral such as calcite to a clay flotation feed and then subjecting the mixture to froth flotation.
The conventional flotation processes have certain inherent disadvantages among which are:
(1) The dispersed feed slurry is best treated at relatively low solids content of 20% by weight, or even 10% by weight or less; hence leading to a large volume of slurry processed per ton of dry clay. For example, increasing the slurry solids from 10% to 30% results in a 74.1% decrease in the amount of water that needs to be removed to produce one ton of clay. Therefore, high levels of slurry solids give rise to lower water removal costs.
(2) The initial froth product contains about 50% of the feed mineral, as well as impurities, which requires reprocessing to obtain reasonable yields of product--four stages of flotation being frequently required.
(3) It is critical that all traces of the carrier mineral, if used, be removed from the beneficiated mineral since the presence of the carrier mineral would be deleterious in the final product.
In some instances in the past, wherein froth flotation has been employed as a part of methods of treating clay to remove color impurities such as titanium mineral impurities, the resulting clay still lacked sufficient brightness. In other instances additional purification techniques such as ultra-flotation techniques employing fine mineral particles (U.S. Pat. No. 3,337,048) or magnetic separation (U.S. Pat. No. 3,974,067) techniques were required for the purpose of achieving an adequate brightness in the treated clay. Such additional processing techniques introduced additional costs and energy consumption. For example, in ultraflotation not only are costs incurred in adding the finely divided mineral particles but costs are also incurred in removing the particles subsequent to froth flotation. The addition and removal of the finely divided particles also results in higher energy consumption. U.S. Pat. No. 3,974,067 illustrates that the addition of a water soluble salt of polyacrylic acid before or during conditioning but not after conditioning and before flotation results in a treated clay which requires the application of further purification techniques in the nature of magnetic separation in order to provide a treating clay of sufficient brightness.
U.S. Pat. No. 2,569,680 to Leek discloses a flotation process for whitening clay which involves treating the clay slip with soda ash and sodium silicate and renders the resulting mixture slightly acid with HCl. The soda ash is intended as a deflocculating agent and sodium silicate is primarily added as a depressant although it is stated that it also assists in deflocculating the clay. The mixture is conditioned and oleic acid is added as collector as well as a flotation agent comprising an emulsion of a fatty acid and a fatty acid amine. The oleic acid is added and the slip is subjected to flotation in which an amyl alcohol mixture is added as frothing agent. The Leek patent, however, fails to disclose or suggest the use of water soluble polyacrylate salt or silicate deflocculants after conditioning and before flotation. Moreover, the patent fails to disclose or suggest the use of an oxidizing bleach before conditioning and a reducing bleach after flotation.
It is also common in some prior art procedures to adjust the pH of the clay slip prior to or during flotation by the use of alkaline agents such as ammonium hydroxide which might be considered to be a deflocculant although not nearly of the nature of silicate and polyacrylate salts.
Other patents such as U.S. Pat. No. 2,740,522 disclose the use of polyacrylate salts such as sodium polyacrylate or ammonium polyacrylate having molecular weights of 10,000 and more and as high as 500,000 molecular weight for the purpose of slime depression. Materials of this type have been known to be strong flocculants when used in the soil conditioning fields but have also been found to be useful in depressing slimes or gangues during the flotation of ores. The patent teaches the addition of polyacrylate salt slime depressants before or during conditioning thus leading the skilled worker away from the present invention in which it is critical to add the polyacrylate salt dispersant after conditioning and before completion of flotation. The patent fails to mention or address itself to the treatment of any clay material. There is no disclosure or suggestion of a clay treatment process which utilizes an oxidizing bleach prior to conditioning and flotation and a reducing bleach after conditioning and flotation. Furthermore, there does not appear to be any disclosure of the use of activating agents. U.S. Pat. No. 3,224,582 uses polyacrylate solid particles during froth flotation for the purpose of acting as collector particles to assist in mechanically stabilizing the foam.
In U.S. Pat. No. 3,599,879 a polyacrylate salt is added to a clay slip during but not after conditioning. Furthermore, the clay slip is diluted after conditioning and before flotation and such dilution results in undesirable energy consumption not only in providing the water for dilution but in removing it when it is desired to isolate or concentrate the clay product. Many other patents including U.S. Pat. Nos. 3,337,048, 3,353,668, 3,450,257 and 3,744,630 all require dilution of the clay slip after conditioning and before flotation. None of the patents mentioned avoid the dilution step resulting in conserving energy.